English

Asymptotic lifting for completely positive maps

Operator Algebras 2025-09-16 v2 Group Theory

Abstract

Let AA and BB be CC^*-algebras with AA separable, let II be an ideal in BB, and let ψ ⁣:AB/I\psi\colon A\to B/I be a completely positive contractive linear map. We show that there is a continuous family Θt ⁣:AB\Theta_t\colon A\to B, for t[1,)t\in [1,\infty), of lifts of ψ\psi that are asymptotically linear, asymptotically completely positive and asymptotically contractive. If ψ\psi is of order zero, then Θt\Theta_t can be chosen to have this property asymptotically. If AA and BB carry continuous actions of a second countable locally compact group GG such that II is GG-invariant and ψ\psi is equivariant, we show that the family Θt\Theta_t can be chosen to be asymptotically equivariant. If a linear completely positive lift for ψ\psi exists, we can arrange that Θt\Theta_t is linear and completely positive for all t[1,)t\in [1,\infty). In the equivariant setting, if AA, BB and ψ\psi are unital, we show that asymptotically linear unital lifts are only guaranteed to exist if GG is amenable. This leads to a new characterization of amenability in terms of the existence of asymptotically equivariant unital sections for quotient maps.

Keywords

Cite

@article{arxiv.2103.09176,
  title  = {Asymptotic lifting for completely positive maps},
  author = {Marzieh Forough and Eusebio Gardella and Klaus Thomsen},
  journal= {arXiv preprint arXiv:2103.09176},
  year   = {2025}
}

Comments

v2: 25 pages

R2 v1 2026-06-24T00:14:38.525Z